This research program examines biological changes underlying the ontogeny of auditory perception. Two major areas of experimentation can be identified. The first series of experiments involves the identification of morphological and physiological changes that occur in the cochlea and brain stem auditory system during the development of hearing. These include: (a) a detailed electron microscopic morphometric study of changes in cochlear structure which can be related to ontogeny of the frequency code along the cochlear partition; (b) an ontogenetic examination of changes in frequency selectivity (the fineness of frequency coding) at the level of the cochlea and at the level of the 8th cranial nerve; and (c) an investigation of the ontogenetic relationships between excitation and inhibition in the central auditory pathways. The second series of experiments uses the brain stem auditory system as a preparation for understanding the development of topographic organization in the central nervous system. Here, we will examine the development of topographic connections between two cell groups and the development of dendritic patterns in the vertebrate central nervous system.